Method for providing magnesium alloys with superplastic properties

ABSTRACT

The invention relates to a method for producing magnesium alloys with a superplastic structure using a deformation process, followed by quick cooling. The magnesium-based alloy is initially pre-heated to a temperature of 340-380° C. and is subsequently deformed. The semi-finished product thus obtained is cooled down at high speed to an ambient temperature immediately after deformation.

[0001] The present invention relates to a method for producing magnesium alloys with a superplastic microstructure.

[0002] By using the superplastic behavior of materials, it is possible to significantly increase the productivity during further processing of semifinished products to form finished components of complex shape compared to conventional working methods. The superplastic shaping of metals and their alloys represents an inexpensive production process in particular for the production of net shape components which can alternatively only be produced by complex machining or joining processes.

[0003] The increasing demand for products which can withstand high loads and are inexpensive to produce, in combination with the ongoing pressure to achieve a lightweight structure, with the associated materials and energy saving, has in recent years produced growing interest in superplastic shaping, in particular in the aeronautical and aerospace industries, in high-speed rail travel, in automotive and mechanical engineering and also in communications and data-processing technology. One reason for this is the particular suitability of this production process for the manufacture of structural components which have the minimum possible wall thickness for lightweight construction.

[0004] While comprehensive investigations into the superplastic behavior of two-phase steels, titanium and aluminum alloys have been carried out, there is only a small amount of fundamental knowledge about the superplastic characteristics of magnesium alloys which, since they have a density which is approximately 50% lower than that of aluminum alloys, could make a further significant contribution to reducing weight in lightweight construction. However, the utilization of the superplastic properties is particularly desirable for the group of materials of the magnesium alloys, on account of their limited cold-workability.

[0005] The term superplasticity is understood as meaning the ability of a material to achieve degrees of deformation which exceed the limits of approximately 10 to 40% which are customary in “normally plastic” materials by a few 100 to over 1000% while only low flow stresses are being applied, without necking and virtually without cold work-hardening. A further feature of the superplastic properties of materials is that the flow stress is highly dependent on the strain rate.

[0006] C. G. Nieh and J. Wadsworth, Scripta Metallurgica et Materialia, Volume 32 (1995), No. 8, pages 1133-1137, describe the production of composite materials comprising ZK60-Mg reinforced with 17% by volume of SiC particles by powder metallurgy processes. According to this article, the presence of the fine SiC particles in ZK60 can apparently refine and stabilize the microstructure of the composite material at high temperatures (450° C.) and is therefore responsible for imparting the superplasticity.

[0007] M. Mabuchi, K. Kubota and K. Higashi, Scripta Metallurgica et Materialia, Volume 33 (1995), No. 2, pages 331-335, describe the production of an Mg-11Si-4Al alloy with a superplastic microstructure by extrusion of “fast-solidified” strips.

[0008] M. Mabuchi, K. Kubota and K. Higashi, Material Transactions, JIM, Vol. 36 (10) (1995) 1249-1254, describe the production of superplastic AZ91 magnesium alloys from machining chips. The chips are produced by machining of a commercial cast ingot of an AZ91 alloy in a lathe and are then extruded. With this alloy, a strain of 980% can be achieved at a working temperature of 573 K and a strain rate of 3.3 10⁻⁴s⁻¹.

[0009] K. U. Kainer, Metall Powder Report 44 (1990), 684-687, describes the production of magnesium alloys with a superplastic microstructure by powder metallurgy processes.

[0010] J. Wolfenstine, G. Gonzalez-Doncel and K. Higashi, Superplasticity and Superplastic Forming (Ed. A. K. Ghosh and T. R. Bieler), 1995, pages 75-82, describe the production of magnesium-lithium alloys with a superplastic microstructure by vacuum-forming and hot-rolling. With this alloy, it is possible to achieve a relative strain of 610% at a working temperature of 350° C. and a strain rate of 4×10⁻⁴s⁻¹.

[0011] J. K. Solberg, J. Torklep, O. Bauger and H. Gjestland, Mater, Sci.Engng. A134 (1991), 1201-1203, finally, describe the production of a superplastic AZ91 magnesium alloy by extremely rapid solidification from the molten state. The alloy had a relative strain of 1480% at 573 K.

[0012] A drawback of the above processes is that the alloys each have to be heated to above the melting point and that the superplastic properties are in each case imparted to the alloys by means of a very complex process (machining/sintering, melting, alloying), making them difficult to employ in particular in industrial processes.

[0013] It is an object of the present invention to provide a method with which, in conventional magnesium-based alloys, it is possible to produce a microstructure with superplastic properties at low cost. The method is to be capable of being used independently of any protective gas techniques customarily employed and is to be capable of simple integration into existing manufacture.

[0014] To achieve this object, the invention proposes a method for providing magnesium alloys with superplastic properties which comprises the following steps:

[0015] a) Heating a magnesium-based alloy to a temperature of 250 and 550° C.,

[0016] b) extruding the cooled alloy to form a semifinished product at a temperature of 200 to 500° C. using a pressing ratio of greater than 1:10,

[0017] c) immediately and rapidly cooling the extrudate which is formed using cooling rates of greater than 300° C./min.

[0018] The base alloy is first of all heated to a temperature of 250 to 600° C., preferably 300 to 450° C. and in particular 340 to 380° C.

[0019] Immediately after the working process, the alloy is subjected to rapid cooling using cooling rates of greater than 300° C./min, more preferably from 500 to 10000° C./min, and in particular of 6000° C./min.

[0020] Without wishing to be tied to any specific theory, it is assumed that the abovementioned rapid cooling immediately after the working process prevents secondary recrystallization in the magnesium alloy, so that an amorphous microstructural state is established. If the material is then exposed to further processing at a temperature of greater than 300° C., which is typical of superplastic working, controlled recrystallization takes place in the amorphous magnesium microstructure, and an extremely fine microstructure is formed, allowing the superplastic deformation properties to be established.

[0021] To achieve the object of allowing the method to be employed independently of any protective gas techniques customarily employed, the invention proposes a method for producing magnesium alloys with a superplastic microstructure which comprises the following steps:

[0022] a) Heating a magnesium-based alloy to a temperature of 300 to 550° C. and holding this temperature for 1 to 100 hours,

[0023] b) cooling the heated alloy to a temperature of 0 to 100° C.,

[0024] c) reheating the cooled alloy to a temperature of 200 to 400° C. and holding this temperature for 1 to 100 hours,

[0025] d) cooling the reheated alloy in air to a temperature of 0 to 100° C.,

[0026] e) working the cooled alloy into a semifinished product at a temperature of 250 to 450° C.

[0027] The base alloy is initially heated slowly, preferably at a heating rate of 0.1 to 3.0° C./min, preferably 0.2 to 1.0° C./min and in particular 0.4 to 0.6° C./min, to a temperature of 300 to 550° C., more preferably 350 to 450° C. and in particularly 390 to 420° C., and is held at this temperature for 1 to 100 hours, preferably 10 to 35 hours and in particular 18 to 24 hours. The heated alloy is then cooled, preferably in air or water, to a temperature of 0 to 100° C., preferably 15 to 50° C. and in particular to ambient temperature.

[0028] In a further step, the cooled alloy is then reheated to a temperature of 200 to 400° C., preferably 220 to 350° C. and in particular 250 to 310° C., and is held at this temperature for 1 to 100 hours, preferably 10 to 35 hours and in particular 18 to 24 hours. The alloy which has been reheated in this way is then cooled, preferably in air or water, to a temperature of 0 to 100° C., preferably 15 to 50° C. and in particular to ambient temperature.

[0029] Preferred magnesium-based alloys for processing in the method according to the invention in addition to magnesium also contain aluminum, zinc, manganese, silicon, copper, zirconium, silver and/or rare earths. Particularly preferred alloys are alloys which contain zinc, zirconium and rare earths, and in particular those which, in addition to magnesium, substantially consist of these elements. Preferred rare earths are neodymium, thorium and yttrium.

[0030] Examples of magnesium-based alloys which can be used are alloys of the following type: AM 20, AM 50, AM 60, AS 41, AS 21, AE 42, AZ 91, EZ 33, AZ 31, QE 22, QH 21, WE 54, ZC 63 and ZRE 1.

[0031] The alloy which has been preheated in accordance with the invention is subjected to conventional working processes in order to convert the alloy into a semifinished product. Preferred working processes are compression-forming processes, such as for example extrusion, rolling or forging. Extrusion is particularly preferred.

[0032] The working of the magnesium alloy by extrusion preferably takes place using a compression ratio of greater than 1:15, more preferably of 1:15 to 1:100, in particular 1:25 to 1:50, with a billet temperature and a receptacle temperature of 200 to 600° C., more preferably 300 to 400° C. In the method described in claim 4, the working of the cooled alloy by extrusion takes place at a billet temperature and a receptacle temperature of 270 to 400° C., preferably 330 to 370° C.

[0033] Without wishing to be tied to a specific theory, it is assumed that the abovementioned heat treatment carried out at below the melting point of the alloy causes finely distributed precipitations to be formed in the microstructure, which during the working operation accumulate at the grain boundaries, where they promote the grain boundary sliding which is characteristic of superplastic deformation. Moreover, precipitations (Mg₁₇Al₁₂, Zr₂Zn₃, Mg₃₂(Al,Zn)₄₉, Mg₉SE) produced by the heat treatment of the various magnesium-based alloys probably act as crystallization nuclei during the secondary crystallization of the microstructure during the working operation.

[0034] According to the manufacturer's specifications, the magnesium-based alloys in the cast state achieve an elongation at break of up to 12%. By contrast, the magnesium-based alloys which are changed to an improved superplastic workability by the inventive method described in claim 1 achieved an elongation at break of up to 550% during tensile tests carried out in a temperature range from 300 to 400° C. with strain rates of 1·10⁻⁴s⁻¹ to 1·10⁻²s⁻¹, while the magnesium-based alloys which had been changed to superplastic workability by the inventive method described in claim 4 reached an elongation at break of up to 780% during tensile tests carried out a temperature of 380° C. and with a constant deformation rate of 0.05 mm/min.

[0035]FIG. 1 shows an undeformed AM20 tensile specimen (a), a tensile specimen of an untreated AM20 magnesium-based alloy which has been deformed under the above conditions (b) and a tensile specimen of an AM20 magnesium-based alloy which has been modified in accordance with Example 1 of the invention and has undergone superplastic deformation under the above conditions (c).

EXAMPLE 1 Production of an AM20 Magnesium-Based Alloy with a Superplastic Microstructure

[0036] A commercially available AM20 magnesium-based alloy was heated to 350° C. Then, the alloy was deformed by extrusion using a compression ratio of 1:29 at a billet temperature of 350° C. and a receptacle temperature of 350° C. Finally, the extrudate emerging from the extrusion die was immediately cooled to 20° C. in a water bath. It was found that the microstructure was in an amorphous state. According to the manufacturer's specifications, in the cast state the base alloy achieves an elongation at break of 12%. By contrast, the magnesium alloy processed using the method according to the invention achieved an elongation at break of 550% (cf. FIG. 1) in tensile tests carried out at a temperature of 380° C. and a deformation rate of 0.6 mm/min.

[0037]FIG. 2 shows an undeformed tensile specimen of an untreated ZRE1 magnesium-based alloy (a), a tensile specimen of an untreated ZRE1 magnesium-based alloy which has been deformed under the above conditions (b) and a tensile specimen of a ZRE1 magnesium-based alloy which has been produced in accordance with Example 2 of the invention and has been superplastically deformed under the above conditions (c).

EXAMPLE 2 Production of a ZRE1 Magnesium-Based Alloy with a Superplastic Microstructure

[0038] A commercially available ZRE1 magnesium-based alloy was slowly heated, at a heating rate of 0.5° C./min, to 415° C. and was held at this temperature for 20 hours. Then, the specimen was cooled in air to ambient temperature. Next, the cooled specimen was overaged by heating to 300° C. and holding the specimen at this temperature for 20 hours. Then, the overaged specimen was cooled again in air to ambient temperature. The specimen was deformed by extrusion with a compression ratio of 1:29 at a billet temperature and receptacle temperature of 350° C. It was found that its microstructure had been refined to a grain size of d=10 μm. According to the manufacturer's specifications, in the cast state the base alloy achieves an elongation at break of 3%. By contrast, the magnesium alloy which had been processed using the method according to the invention achieved an elongation at break of 780% (cf. FIG. 2) at a temperature of 380° C. and a deformation rate of 0.05 mm/min. 

1. A method for producing magnesium alloys with a superplastic microstructure, which comprises the following steps: a) Heating a magnesium-based alloy to a temperature of 250 to 600° C., b) working the cooled alloy to form a semifinished product at a temperature of 250 to 450° C., c) immediately and rapidly cooling the semifinished product using cooling rates of greater than 300° C./min.
 2. The method as claimed in claim 1, characterized in that in step a) the alloy is heated to a temperature of 340 to 380° C.
 3. The method as claimed in claim 1 or 2, characterized in that the extrudate is quenched immediately at a cooling rate of 6000° C./min.
 4. A method for producing magnesium alloys having a superplastic microstructure, which comprises the following steps: f) Heating a magnesium-based alloy to a temperature of 300 to 550° C. and holding this temperature for 1 to 100 hours, g) cooling the heated alloy to a temperature of 0 to 100° C., h) reheating the cooled alloy to a temperature of 200 to 400° C. and holding this temperature for 1 to 100 hours, i) cooling the reheated alloy in air to a temperature of 0 to 100° C., j) working the cooled alloy to form a semifinished product at a temperature of 250 to 450° C.
 5. The method as claimed in claim 4, characterized in that in step a) the alloy is heated to a temperature of 390 to 420° C.
 6. The method as claimed in claim 4 or 5, characterized in that in step c) the alloy is heated to a temperature of 250 to 310° C.
 7. The method as claimed in claim 4, 5 or 6, characterized in that in step b) and/or d) the alloy is cooled to ambient temperature.
 8. The method as claimed in one of claims 4 to 7, characterized in that in step a) and/or b) the heated alloy is held at the final temperature for 12 to 24 hours.
 9. The method as claimed in one of the preceding claims, characterized in that the magnesium-based alloy in addition to magnesium also contains aluminum, zinc, manganese, silicon, copper, zirconium, silver and/or rare earths.
 10. The method as claimed in one of the preceding claims, characterized in that the alloy is worked by extrusion.
 11. The method as claimed in claim 10, characterized in that the pressing ratio during the extrusion is 1:25 to 1:50.
 12. The method as claimed in claim 10 or 11, characterized in that the billet temperature and the receptacle temperature during the extrusion is 300 to 400° C. 